The present invention relates to an electroluminescent device (referred as an EL device hereinafter), and more particularly to an EL device wherein an organic thin film is provided between a polycrystalline thin film made of a II-VI compound and an electrode.
The study of EL devices has been vigorously pursued as a result of the need for making electrical and electric equipment small, light and thin, and improving the quality of they display. Recently, there have been developed and commercialized thin film EL devices obtained by sandwiching a Mn-doped ZnS luminescent layer between insulating or dielectric layers, in the so-called double insulating layer structure. Though these devices have high brightness and long life, a practical problem has remained, namely the high AC driving voltage of about 200 V due to the existing insulating layers. These devices are very expensive, because IC which has high withstand voltage have to be custom made and employed in such devices.
There has been desired the development of thin film EL devices which can be driven under low voltage in order to solve the above-mentioned problem and whereby to simplify driving circuits and reduce the cost. For this purpose, there has been reported the possibility of reducing the operational voltage to about 60 V by using ferroelectrics such as lead titanate (Japanese Journal of Applied Physics Vol. 20 (1981) Supplement 20-1 pp 215-220). However, the desirable devices which can be driven at the voltage of not more than 50 V have not been realized, so the EL devices have not been widely used due to high cost. Further, DC driving EL devices having MIS (Metal/Insulating Layer/Semiconductor) structure or M.pi.S (Metal/Semiinsulating-layer/semiconductor) structure have been briskly studied.
There have been developed blue-light emitting EL devices, for example, wherein single crystalline ZnS or ZnSe is epitaxially grown on bulk single crystals such as ZnS, ZnSe, GaP or GaAs. Then there is formed an insulating layer or semi-insulating layer of ZnO or ZnS thereon by heat-treatment, acid-treatment, evaporation or MOCVD (Metal-Organic Chemical Vapor Deposition) method, and the like.
A group including Dr. Roberts of Durham University has been studying MIS EL devices wherein Langmuir Blodgett films are deposited on ZnS or ZnSe single crystalline thin films which are epitaxially grown on n-GaP single crystals in order to obtain blue-light emitting EL devices.
In case of using bulk ZnS or ZnSe single crystals, it is difficult at this stage to make large-area single crystals suitable for EL devices of large area, and accordingly the use of bulk ZnS or ZnSe single crystal is only examined at the laboratory.
On the other hand, in case of using epitaxially grown single crystalline thin films on n-GaP single crystals, and the like, the above-mentioned problem is to some extend solved. However, it is practically difficult to produce large-area devices of, for example, 200.times.200 mm because the size of such devices is determined by the size of single crystalline wafers used. Accordingly the cost of such devices is high.
It is an object of the present invention to remove the above-mentioned drawbacks by providing an electroluminescent device wherein an organic thin film is provided between a polycrystalline thin film made of II-VI compound and an electrode. The device of the present invention can radiate at low voltage and with high brightness and be obtained at low production cost and in a large area.